This invention relates generally to the processing of composite video signals and is particularly directed to the generation of selected field or frame rate signals in response to a composite video signal.
The non-video, or retrace, portion of composite video signals is increasingly being used for the transmission of data. For example, some cable television (CATV) systems use the vertical blanking interval (VBI) to transmit "downstream" data from the system headend to individual subscribers. This data may include subscriber address information, viewer authorization instructions, subscriber polling signals, and video signal control data relating to such signal parameters as sync suppression and video signal inversion. In order for this data to be accurately detected, it must be transmitted in precise timed relation with respect to the vertical sync interval.
To ensure proper timing of the transmitted and received data the communications system must be capable of accurately detecting the lines during which the data is transmitted. In the NTSC system, the vertical or field rate is equal to 262.5 horizontal lines, with two interlaced fields comprising each frame. Prior art approaches for generating a vertical or field rate signal generally involve dividing a 2X horizontal rate (2H) signal by 525. Since two fields form one frame, frame rate is obtained by dividing vertical by 2. This approach involves the use of complicated counting arrangements for generating these vertical reference signals.
The present invention represents an improvement over the prior art by providing a simplified arrangement for precisely defining pulses at field or frame rates to be used in the processing of a composite video signal. The present invention makes use of timing signals generated by horizontal and vertical processing circuitry for precisely defining leading and trailing edges of the thus generated vertical pulses.